1. Field of the Invention
The present invention relates to a control device of a vehicle provided with a power train having an engine and an automatic transmission, and particularly to a control device of a vehicle that can achieve deceleration matching with a driver's preference.
2. Description of the Background Art
In generally, an automatic transmission connected to an engine of a vehicle is automatically controlled by determining a transmission gear ratio depending on an operation amount of an accelerator pedal by a driver and a vehicle speed. During ordinary driving, the gear ratio is increased with decrease in accelerator pedal operation amount or with increase in vehicle speed.
Therefore, during downhill driving or the like, a driver releases the accelerator pedal to decrease the accelerator pedal operation amount, and the gear ratio is controlled to increase so that the control is performed to decrease an engine braking effort of the vehicle. Thereby, the driver may feel unnecessary acceleration, and/or may operate a brake more times so that acceleration feeling requested by the driver cannot be achieved in some cases.
Japanese Patent Laying-Open No. 10-095252 has disclosed a vehicle drive power control device that changes an engine braking force in response to a driver's manual operation even when automatic engine brake control is active during downhill driving. This control device controls a drive force of a vehicle provided with an engine and an automatic transmission connected to the engine, and operates as follows. When a throttle is fully closed, a target deceleration resistance that matches with a target deceleration corresponding to the vehicle speed is set depending on a transmission range selected in an automatic transmission, calculation is performed to obtain a running resistance externally applied to the vehicle and a target drive resistance of the vehicle to be added to the running resistance for obtaining a target deceleration resistance, calculation is performed to obtain a range or area that can generate a vehicle drive resistance depending on the gear ratio of the automatic transmission, the gear ratio of the automatic transmission is controlled to keep the target drive resistance in the range that can generate the drive resistance, and a negative output of the engine is controlled to generate the target drive resistance with the controlled gear ratio.
According to this vehicle drive power control device, when the throttle is fully closed, the target deceleration resistance (=(vehicle mass)×(target deceleration)) that matches with the target deceleration corresponding to the vehicle speed is set depending on the transmission range selected in the automatic transmission. Also, the calculation is performed to obtain the running resistance that is externally applied to the vehicle and includes a gradient resistance, an air resistance and/or a rolling resistance, and the target drive resistance of the vehicle to be added to the running resistance is calculated to satisfy the target deceleration resistance. Although a relationship of ((target deceleration resistance)=(running resistance)+(target drive resistance)) is established, the running resistance takes a negative value (assuming that the deceleration direction is positive) when a descent gradient is large so that the target drive resistance becomes larger in absolute value than the target deceleration resistance by the magnitude of the running resistance. The vehicle drive resistance changes within a predetermined range between the maximum and minimum values under control of the negative output of the engine, and this predetermined range changes according to the gear ratio of the automatic transmission. Therefore, calculation is performed to obtain a range that can generate the vehicle drive resistance corresponding to the gear ratio of the automatic transmission, and the gear ratio of the automatic transmission is controlled such that the target drive resistance thus calculated may enter the range allowing generation of the vehicle drive resistance. As described above, the gear ratio is controlled to keep the target drive resistance within the range that can generate the vehicle drive resistance as described, and further the negative output of the engine is controlled to provide the drive resistance of the vehicle matching with the target drive resistance. Thereby, the target drive resistance is obtained, and the vehicle is decelerated with the target deceleration. When a driver changes a transmission range of the automatic transmission with an operation lever or the like, a new target deceleration resistance is set, and thereby the drive resistance of the vehicle is controlled to change according to the change in target drive resistance. Therefore, the engine braking force can reliably change in such an expected manner that the engine braking force changes according to the transmission range selection of the driver.
Although a vehicle drive force control device disclosed in Japanese Patent Laying-Open No. 10-095252 can achieve the target drive resistance, there is doubt about whether the achieved target drive resistance matches with the deceleration requested by the driver or not. More specifically, there is a technique that a driver's preference (sporty preference allowing rapid acceleration/deceleration and a large lateral G (Gravity), or a comfort preference for enjoying gentle driving) is estimated or is set by the driver to achieve the vehicle's behavior matching with the driver's preference. Since such driver's preference is not reflected in the calculation of the target drive resistance, the vehicle drive force control device disclosed in Japanese Patent Laying-Open No. 10-095252 does not reflect the driver's preference. Consequently, the driver's preference is not reflected in the deceleration operation, and the driver feels the deceleration caused by unexpected engine braking.